FIG. 1 illustrates a schematic diagram of a frame structure in a TDD (Time Division Duplex) mode of a LTE system, which is also called frame structure type 2. In this type of frame structure, a radio frame of 10 ms (307200 Ts, 1 ms=30720 Ts) is divided into two half-frames, each having a length of 5 ms (153600 Ts) and containing 5 subframes with the length of 1 ms. The role of each of the subframes is shown in Table 1, where D represents a downlink subframe used for transmitting downlink signals, and U represents an uplink subframe (which is called generic uplink subframe) used for transmitting uplink signals. Furthermore, an uplink/downlink subframe is divided into 2 time slots of 0.5 ms, and S represents a special subframe which contains three special time slots, a DwPTS (Downlink Pilot Time Slot), a GP (Guard Period) and an UpPTS (Uplink Pilot Time Slot). In an actual system, indices of uplink/downlink configuration will be notified to a UE (User Equipment) through a broadcast message.
TABLE 1uplink/downlink configurationConfigurationConfiguration indexSwitch-point periodicitySubframe numbernumberSwitch-point periodicity012345678905msDSUUUDSUUU15msDSUUDDSUUD25msDSUDDDSUDD310msDSUUUDDDDD410msDSUUDDDDDD510msDSUDDDDDDD65msDSUUUDSUUD
Resource allocation in the LTE system takes a PRB (Physical Resource Block, or Resource Block (RB) for short) as a unit, and one PRB occupies 12 subcarriers (which are also called Resource Elements (RE), each subcarrier being 15 kHz) in frequency domain and occupies one time slot in time domain, that is, it is a SC-FDMA symbol occupying 7 normal cyclic prefixes (Normal CP) or 6 extended cyclic prefix (Extended CP) in time domain. If the total number of RBs corresponding to uplink system bandwidth in frequency domain is NRBUL, then indices of the RBs are 0, 1, . . . , NRBUL−1, and indices of REs are 0, 1, . . . , NRBUL·NSCRB−1, where NSCRB is the number of subcarriers corresponding to one RB in frequency domain. Taking the normal cyclic prefixes as an example, the structure of the PRB is shown in FIG. 2.
A physical random access channel (PRACH) may also be called a random access opportunity or random access resource, and one random access channel corresponds to one random access preamble, which consists of two portions, a cyclic prefix (CP) and a sequence, as shown in FIG. 3. Different random access preamble formats mean different CP and/or sequence lengths. Currently, types of the preamble formats that are supported by the TDD mode in the LTE system are shown in FIG. 2.
TABLE 2Preamble FormatTCPTSEQ0 3168 · TS    24576 · TS121024 · TS    24576 · TS2 6240 · TS2 · 24576 · TS321024 · TS2 · 24576 · TS4 448 · TS    4096 · TS(only for frame structure type 2)
In the random access preamble formats shown in FIG. 2, preamble formats 0˜3 are transmitted in the generic uplink subframe, while preamble format 4 is transmitted in the UpPTS. In frequency domain, one random access preamble occupies a bandwidth corresponding to 6 PRBs, that is, 72 REs. PRACHs with the same time domain position are differentiated through the frequency domain.
In the TDD mode of the LTE system, random access configurations are shown in Table 3, where DRA the density of the random access channels, when DRA>0.5, it represents the number of PRACHs per radio frame of 10 ms, and when DRA=0.5, it means 20 ms, i.e., there is one PRACH per 2 radio frames; rRA is a version number corresponding to a PRACH configuration index. A base station informs a terminal of a PRACH configuration index number such that the terminal can obtain a parameter corresponding to the index number.
TABLE 3DensityPRACHPreamblePer 10 msVersionconf. IndexFormat(DRA)(rRA)000.50100.51200.523010401150126020702180229030100311103212040130411404215050160511705218060190612010.502110.512210.52231102411125120261302714028150291603020.503120.513220.52332103421135220362303724038250392604030.504130.514230.5243310443114532046330473404840.504940.515040.5251410524115342054430554405645057460
A sounding reference signal (SRS) is used for measuring uplink channel quality. Subcarriers of the SRS in the same SRS frequency band are positioned at intervals, as shown in FIG. 4. Such a comb-like structure allows more users to send the SRS in the same SRS bandwidth. The bandwidth of the SRS is configured using a tree-like structure, that is, each SRS bandwidth configuration corresponds to one tree-like structure, wherein SRS bandwidth in the top layer corresponds to the maximum bandwidth of this SRS bandwidth configuration. Table 4˜Table 7 show SRS bandwidth configurations in different uplink system bandwidth ranges respectively.
Take SRS bandwidth configuration 1 in Table 4 as an example, b=0 is the first layer, the top layer of the tree-like structure, the SRS bandwidth of which is a bandwidth corresponding to 32 PRBs and is the maximum SRS bandwidth of this SRS bandwidth configuration; b=1 is the second layer, the SRS bandwidth of which is a bandwidth corresponding to 16 PRBs, and one SRS bandwidth of the previous layer is divided into 2 SRS bandwidths (Nb=2) in the second layer; b=2 is the third layer, the SRS bandwidth of which is a bandwidth corresponding to 8 PRBs, and one SRS bandwidth of the previous layer is divided into 2 SRS bandwidths of the third layer; and b=3 is the fourth layer, the SRS bandwidth of which is a bandwidth corresponding to 4 PRBs, and one SRS bandwidth of the previous layer is divided into 2 SRS bandwidths of the fourth layer.
TABLE 4(6 ≦ NRBUL ≦ 40)SRS-BandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthSRS bandwidthb = 0b = 1b = 2b = 3configurationmSRS, bNbmSRS, bNbmSRS, bNbmSRS, bNb03611234341132116282422241464141320145414141614441415121434141681424141741414141
TABLE 5(40 ≦ NRBUL ≦ 60)SRS-BandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthSRS bandwidthb = 0b = 1b = 2b = 3configurationmSRS, bNbmSRS, bNbmSRS, bNbmSRS, bNb04812421224314811638242240120245413361123434143211628242524146414162014541417161444141
TABLE 6(60 ≦ NRBUL ≦ 80)SRS-BandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthSRS bandwidthb = 0b = 1b = 2b = 3configurationmSRS, bNbmSRS, bNbmSRS, bNbmSRS, bNb0721243122431641322162442601203454134812421224344811638242540120245416361123434173211628242
TABLE 7(80 ≦ NRBUL ≦ 110)SRS-BandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthSRS bandwidthb = 0b = 1b = 2b = 3configurationmSRS, bNbmSRS, bNbmSRS, bNbmSRS, bNb0961482242461961323162442801402202453721243122434641322162445601203454164812421224374811638242
A SRS may be transmitted in the UpPTS, and then the first carrier in a frequency band range of the SRS is determined according to the following formula.
      k    0    ′    =      {                                                                      (                                                      N                    RB                    UL                                    -                                      m                                          SRS                      ,                      0                                                                      )                            ⁢                              N                sc                RB                                      +                          k              TC                                                                          if              ⁢                                                          ⁢                              (                                                                            (                                                                        n                          f                                                ⁢                        mod                        ⁢                                                                                                  ⁢                        2                                            )                                        ×                                          (                                              2                        -                                                  N                          SP                                                                    )                                                        +                                      t                    RA                    1                                                  )                            ⁢              mod              ⁢                                                          ⁢              2                        =            0                                                            k            TC                                    otherwise                    where NRBUL is the number of RBs corresponding to the uplink system bandwidth, mSRS,0 is the frequency band range of the SRS (maximum SRS bandwidth), NSCRB is the number of subcarriers in one RB, kTC is a start point of the comb-like structure, and kTCε{0,1}, nf is a system frame number (SFN) of a radio frame in which the UpPTS is located, NSP is the number of downlink-to-uplink switch-points in one radio frame, tRA1=0 and tRA1=1 represent the first half-frame and the second half-frame respectively, that is to say, tRA1=0 when the UpPTS is in the first half-frame of the radio frame, and tRA1=1 when UpPTS is in the second half-frame of the radio frame.
Additionally, when the SRS bandwidth is selected such that b=0, the SRS bandwidth may be reconfigured asmSRS,0=maxcεC{mSRS,0c}≦{NRBUL−6NRA}where c is a SRS bandwidth configuration, C is a set of bandwidth configurations in Table 4˜Table 7, NRA is the number of PRACHs contained in the UpPTS in which the SRS is located. That is to say, when b=0, the SRS bandwidth equals to the maximum SRS bandwidth which is less than or equal to NRBUL−6NRA in a set of all SRS bandwidth configurations (including those in Table 4˜Table 7) in all uplink system bandwidth ranges. In another instance, the SRS bandwidth may also be reconfigured as the maximum SRS bandwidth which is less than or equal to NRBUL−6NRA in a set of all SRS bandwidth configurations (including those in only one table) in the uplink system bandwidth range which contains the current uplink system bandwidth.
When the SRS bandwidth is reconfigured, the number of PRACHs in the UpPTS is required to be known. The conventional method is to combine all PRACH configuration states with all uplink-to-downlink proportion configurations to traverse the number of channels in an UpPTS in each state, and store the resulting result into a table. This method needs to store such a table in both the base station and the terminal, thus system overhead is very large.